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We derive the double recurrence $e_n = \frac{1}{2}(a_{n-1}+5b_{n-1}); f_{n} = \frac{1}{2}(a_{n-1}+b_{n-1})$ with $e_0=2;f_0=0$ for the Fibonacci numbers, leading to an extremely simple and fast implementation. Though the recurrence is probably not new, we have not been able to find a reference for it.

In this paper we present quantum key distribution protocol that, instead of single qubits, uses mesoscopic coherent states of light $|\alpha\rangle$ to encode bit values of a randomly generated key. Given the reference value $\alpha\in\mathbb C$, and a string of phase rotations each randomly taken from a set of $2M$ equidistant phases, Alice prepares a quantum state given by a product of coherent states of light, such that a complex phase of each pulse is rotated by the corresponding phase rotation. The encoding of $i$-th bit of the key $r=r_1 \dots r_\ell$ is done by further performing phase rotation $r_i \pi$ (with $r_i = 0,1$) on the $i$-th coherent state pulse. In order to protect the protocol against the man-in-the-middle attack, we introduce a verification procedure, and analyse the protocol's security using the Holevo bound. We also analyse the possibility of beam splitting-like and of collective attacks, showing the impossibility of the former and, in the case of our protocol, the inadequacy of the latter. While we cannot prove full perfect security against the most general attacks allowed by the laws of quantum mechanics, our protocol achieves faster quantum key distribution, over larger distances and with lower costs, than the single-photon counterparts, maintaining at least practical security against the current and the near future technologies., Comment: 8 pages, 2 figures

One-time-pad (OTP) encryption simply cannot be cracked, even by a quantum computer. The need of sharing in a secure way supplies of symmetric random keys turned the method almost obsolete as a standing-alone method for fast and large volume telecommunication. Basically, this secure sharing of keys and their renewal, once exhausted, had to be done through couriers, in a slow and costly process. This paper presents a solution for this problem providing a fast and unlimited renewal of secure keys: An untappable key distribution system is presented and detailed. This fast key distribution system utilizes two layers of confidentially protection: 1) Physical noise intrinsic to the optical channel that turn the coded signals into stealth signals and 2) Privacy amplification using a bit pool of refreshed entropy run after run, to eliminate any residual information. The resulting level of security is rigorously calculated and demonstrates that the level of information an eavesdropper could obtain is completely negligible. The random bit sequences, fast and securely distributed, can be used to encrypt text, data or voice., Comment: 12 pages, 16 figures

The dining cryptographers protocol provides information-theoretically secure sender and recipient untraceability. However, the protocol is considered to be impractical because a malicious participant may disrupt the communication. We propose an implementation which provides information-theoretical security for senders and recipients, and in which a disruptor with limited computational capabilities can easily be detected., Comment: 12 pages

Security of quantum key distribution against sophisticated attacks is among the most important issues in quantum information theory. In this work we prove security against a very important class of attacks called collective attacks (under a compatible noise model) which use quantum memories and gates, and which are directed against the final key. Although attacks stronger than the collective attacks can exist in principle, no explicit example was found and it is conjectured that security against collective attacks implies also security against any attack., Comment: 5 pages, RevTeX

This paper, mostly expository in nature, surveys four measures of distinguishability for quantum-mechanical states. This is done from the point of view of the cryptographer with a particular eye on applications in quantum cryptography. Each of the measures considered is rooted in an analogous classical measure of distinguishability for probability distributions: namely, the probability of an identification error, the Kolmogorov distance, the Bhattacharyya coefficient, and the Shannon distinguishability (as defined through mutual information). These measures have a long history of use in statistical pattern recognition and classical cryptography. We obtain several inequalities that relate the quantum distinguishability measures to each other, one of which may be crucial for proving the security of quantum cryptographic key distribution. In another vein, these measures and their connecting inequalities are used to define a single notion of cryptographic exponential indistinguishability for two families of quantum states. This is a tool that may prove useful in the analysis of various quantum cryptographic protocols., Comment: 28 pages, 3 figures, minor typographical changes, submitted to IEEE Transactions on Information Theory

We have investigated experimentally the electronic transport properties of a two-dimensional electron gas (2DEG) present in an AlSb/InAs/AlSb quantum well, where part of the toplayer has been replaced by a superconducting Nb strip, with an energy gap $\Delta_0$. By measuring the lateral electronic transport underneath the superconductor, and comparing the experimental results with a model based on the Bogoliubov-de Gennes equation and the Landauer-B\"uttiker formalism, we obtain a decay length $\xi_{\text{Sm}} \approx 100~\text{nm}$ for electrons. This decay length corresponds to an interface transparency $T_{\text{SIN}}=0.7$ between the Nb and InAs. Using this value, we infer an energy gap in the excitation spectrum of the SQW of $\Delta_{\text{eff}} = 0.97 \Delta_0 = 0.83~\text{meV}$., Comment: Revtex, 3 PostScript figures